Problem: Simplify the following expression: $q = \dfrac{1}{2y} - \dfrac{1}{5y}$
In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $2y$ and $5y$ $\lcm(2y, 5y) = 10y$ $ q = \dfrac{5}{5} \cdot \dfrac{1}{2y} - \dfrac{2}{2} \cdot \dfrac{1}{5y} $ $q = \dfrac{5}{10y} - \dfrac{2}{10y}$ $q = \dfrac{5 -2}{10y}$ $q = \dfrac{3}{10y}$